U.S. patent number 6,681,710 [Application Number 10/075,543] was granted by the patent office on 2004-01-27 for system for controlling a marine seismic array.
This patent grant is currently assigned to Petroleum Geo-Services AS. Invention is credited to Fredrik Ole Semb.
United States Patent |
6,681,710 |
Semb |
January 27, 2004 |
System for controlling a marine seismic array
Abstract
System for controlling seismic arrays comprising at least one
deflector coupled to one side of the towing vessel through at least
one wire, lead-in or similar, the detector being positioned at a
distance perpendicular to the vessel's direction of movement, the
vessel being provided with a navigation system for measuring the
position of the vessel. The vessel comprises calculation means for,
on the basis of the vessel's position, calculating deviations in
vessel's position from a predetermined path. The wire is coupled to
the vessel through control organs, e.g. a winch, adapted to vary
the wire length from the vessel to the deflector. The control
organs are coupled to the calculation means for adjusting the wire
length based on the deviations in the position of the towing
vessel, thus to avoid corresponding deviations in the movements of
the deflector.
Inventors: |
Semb; Fredrik Ole (Holmestrand,
NO) |
Assignee: |
Petroleum Geo-Services AS
(Lysaker, NO)
|
Family
ID: |
19903669 |
Appl.
No.: |
10/075,543 |
Filed: |
February 14, 2002 |
Current U.S.
Class: |
114/244;
114/253 |
Current CPC
Class: |
G01V
1/3817 (20130101); B63B 21/66 (20130101) |
Current International
Class: |
B63B
21/56 (20060101); B63B 21/66 (20060101); G01V
1/38 (20060101); B63B 021/66 () |
Field of
Search: |
;114/242,247,253 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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|
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|
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|
|
2047406 |
|
Nov 1980 |
|
GB |
|
2087556 |
|
May 1982 |
|
GB |
|
2184413 |
|
Jan 1987 |
|
GB |
|
Primary Examiner: Sotelo; Jesus D.
Attorney, Agent or Firm: Thigpen; E. Eugene
Parent Case Text
This application claims priority to Norwegian patent application
Serial No. 1999.3971, filed Aug. 17, 1999.
Claims
What is claimed is:
1. A system for controlling a marine seismic array comprising at
least one deflector at one side of a towing vessel coupled through
a towing cable, said at least one deflector being placed in a first
position at a distance perpendicular to a direction of movement of
the towing vessel, the towing vessel being equipped with a
navigation system for measuring a second position of said towing
vessel, characterized in that the towing vessel comprises
calculation means for calculating deviations in movement of the
towing vessel relative to a predetermined course; that said towing
cable is coupled to the towing vessel through a control means for
varying towing cable length from the towing vessel to said at least
one deflector; that the control means are coupled to the
calculation means for adjusting said towing cable length depending
on said deviations in said second position, thus to counteract
corresponding deviations in movement of said at least one
deflector.
2. The system according to claim 1, characterized in that a
deviation in said second position perpendicular to said direction
of movement is compensated by the control means by adjusting said
towing cable length with a length corresponding to the deviation
divided by sine of an angle between a line from the control means
to said at least one deflector and the predetermined course.
3. The system according to claim 1, characterized in that a
deviation in said second position relative to said direction of
movement is compensated by the control means by adjusting said
towing cable length with a length corresponding to the deviation
divided by cosine of an angle between a line from the control means
to the said at least one deflector and the predetermined
course.
4. The system according to claim 1, characterized in that the
towing vessel comprises means for measuring said first position of
said at least one deflector relative to the towing vessel, and that
compensation for deviations in said first position comprises
controlling a global position of said at least one deflector based
on said second position and said first position.
5. The system according to claim 1, characterized in that said
calculating means also comprises means for measuring deviations in
roll, pitch, and heave of the towing vessel, and said control means
also are adapted to compensate for said deviations in roll, pitch,
and heave.
6. The system according to claim 1, characterized in that said
marine seismic array comprises two deflectors, one on each side of
the towing vessel relative to said direction of movement of the
towing vessel, each of said two deflectors being coupled to said
control means and where said marine seismic array is spanned
between said two deflectors.
7. The system according to claim 1, characterized in that said
control means comprises means for adjusting a lift capability of
said at least one deflector for adjusting said first position.
8. A method for controlling a marine seismic array comprising at
least one deflector at one side of a towing vessel coupled through
a towing cable, said at least one deflector being placed in a first
position with a distance perpendicular to a direction of movement
of the towing vessel, the towing vessel being equipped with a
navigation system for measuring a second position of the towing
vessel, characterized in calculating deviations in movements of the
towing vessel relative to a predetermined course, and varying
length of said towing cable from the towing vessel to said at least
one deflector depending on said deviations, thus to counteract
corresponding deviations in movements of the at least one
deflector.
9. The method according to claim 8, characterized in that
compensation for a deviation in said second position perpendicular
to said direction of movement is performed by varying towing cable
length with a length corresponding to said deviation in said second
position divided by sine of an angle between a line from said
control means to said at least one deflector and said predetermined
course.
10. The method according to claim 8, characterized in that
compensation for a deviation in said second position relative to
said direction of movement is performed by varying towing cable
length with a length corresponding to said deviation in said second
position divided by cosine of an angle between a line from said
control means to said at least one deflector and said predetermined
course.
11. The method according to claim 8, characterized in measuring
said first position of said at least one deflector relative to the
towing vessel, and controlling a global position of said at least
one deflector based on said second position and said first position
relative to the towing vessel.
12. The method according to claim 8, characterized in measuring
deviations in roll, pitch, and heave of towing vessel, and
compensating for said deviations in roll, pitch, and heave by
varying towing cable length.
13. The method according to claim 8, characterized in adjusting a
lift capability of said at least one deflector to adjust said first
position.
14. A system for controlling movement of a seismic array in tow by
a marine vessel, with said seismic array having deflectors coupled
by way of towing cables to said marine vessel, and said marine
vessel having a navigation system, which comprises: calculating
means in communication with said navigation system and receiving
position of said marine vessel from said navigation system, and
calculating the refrom deviations of said position from a
predetermined path; and control means connected to said towing
cables and in communication with said calculating means for
adjusting length of said towing cables to compensate for said
deviations and thereby maintain said deflectors on said
predetermined path.
15. The system of claim 14, wherein said seismic array is
symmetrical with at least one deflector on each side of said marine
vessel, said at least one deflector being coupled to said marine
vessel by means of towing cables, and said seismic array further
including seismic sensors coupled to said towing cables by means of
seismic cables, and said seismic cables being connected in parallel
a distance apart by means of transverse cables located between said
towing cables.
16. The system of claim 15, wherein said control means adjusts
length of said seismic cables to compensate for said
deviations.
17. The system of claim 15, wherein said control means adjusts
length of both said towing cables and said seismic cables to
compensate for said deviations.
18. The system of claim 14, wherein said navigation system is a
Global Positioning System.
19. The system of claim 14, wherein said navigation system
calculates positions of said deflectors globally and relative to
said marine vessel, and calculates said position of said marine
vessel, and wherein said control means compensates for said
deviations by adjusting length of said towing cables to maintain
said deflectors parallel to said predetermined path.
20. The system of claim 14, wherein for those of said deviations
which are perpendicular to said predetermined path, said control
means adjusts length of said towing cables in accordance with a
formula .DELTA.k=a.perp./sin .theta., where a.perp. is a
perpendicular deviation which is perpendicular to said
predetermined path, .DELTA.k is a change in cable length, and
.theta. is an angle between said predetermined path and a line from
said control means to one of said deflectors.
21. The system of claim 20, wherein said control means adjusts
length of all of said towing cables at a same time.
22. The system of claim 14, wherein for those of said deviations
which are parallel to said predetermined path, said control means
adjusts length of said towing cables in accordance with a formula
.DELTA.k=a.sub..vertline. /cos.theta., where a.sub..vertline. is a
parallel deviation which is parallel to said predetermined path,
.DELTA.k is a change in cable length, and .theta. is an angle
between said predetermined path and a line from said control means
to one of said deflectors.
23. The system of claim 22, wherein said control means adjusts
length of all of said towing cables at a same time.
24. The system of claim 14, wherein said deviations include both
deviations perpendicular to said predetermined path and deviations
parallel to said predetermined path, and said control means adjusts
length of all of said towing cables at a same time.
25. The system of claim 14, wherein said deflectors are active
deflectors and said control means adjusts lift of said deflectors
to compensate for said deviations.
26. The system of claim 14, wherein said deviations include heave,
roll, and pitch as well as deviations perpendicular to said
predetermined path and deviations parallel to said predetermined
path.
27. The system of claim 14, wherein said seismic array is comprised
of plural parts, each part being coupled to at least one of said
deflectors, and said control means adjusting length of said towing
cables to position said plural parts to maintain said seismic array
on said predetermined path.
28. The system of claim 14, wherein said seismic array is
asymmetrical.
29. The system of claim 14, wherein said control means is a
winch.
30. A method for controlling movement of a seismic array having
deflectors for maintaining span of said seismic array, and being in
tow by a marine vessel having a navigation system, a calculating
means in communication with said navigation system, and a control
means responsive to said calculating means, which comprises
following steps: said navigation system determining position of
said marine vessel; said calculating means receiving position of
said marine vessel from said navigation system; said calculating
means determining deviations in said position relative to a
predetermined path; and said control means varying length of towing
cables connecting said control means to said deflectors to
compensate for said deviations, and thereby maintain said
deflectors along a path parallel to said predetermined path.
31. The method of claim 30, wherein said navigation system is a
Global Positioning System, and said control means is a winch.
32. The method of claim 30, wherein step of determining deviations
in said position includes calculating deviations perpendicular to
said predetermined path, and calculating deviations parallel to
said predetermined path.
33. The method of claim 30, wherein said control means is varying
length of seismic cables connected to said towing cables.
34. The method of claim 33, wherein said control means is varying
length of said towing cables to compensate for said deviations, and
varying length of said seismic cables to compensate for relative
displacement in direction of movement of said marine vessel.
35. The method of claim 30, wherein said deflectors are active
deflectors, and said control means is varying lift of said active
deflectors to compensate for said deviations.
36. The method of claim 30, wherein said deviations are deviations
in pitch, roll, and heave of said marine vessel.
Description
This invention relates to a system and a method for controlling a
towed seismic array comprising at least one deflector on one side
of a towing vessel coupled through at least one wire, lead-in or
similar, the deflector being positioned with a distance
perpendicular to the towing vessels direction of movement, the
towing vessel being provided with a navigation system for measuring
the position of the vessel.
In seismic surveys at sea a number of seismic cables are usually
being towed after a vessel. The survey of the geological formations
at the sea bottom is performed by sending sound waves from one or
more acoustic sources down into the sea bottom where they are
reflected at the transitions between different types of formations.
The reflected signals are received by sensors positioned in the
seismic cables. The towed cable array is towed along a chosen path
to perform the survey in a chosen area. The movements of the vessel
and the array must be controlled precisely to secure a coverage of
the wanted areas.
Accurate control of the vessels and arrays positions are especially
important when the same reflection points are to be surveyed more
than once to improve the measurements. This may be done by
controlling the time between the emitted acoustic signals relative
to the distance between the sensors and the vessels velocity so
that the next signal is reflected from the reflection point up to a
later sensor along the same cable.
To secure accurate measurements it is usual to monitor the position
of the vessel using existing navigation systems to correct this. It
has, however, become evident that it is difficult to obtain the
required accuracy, among other reasons because of the size and
momentum of the vessels. Different systems have also been used for
compensating for the errors, e.g. signal treatment, over sampling
or by controlling the deflectors pulling the array sideways out
from the vessel, thus increasing the complexity of the system, e.g.
as the control signals and power has to be transmitted from the
vessel out to the deflectors.
U.S. Pat. No. 4,781,140 describes a system for compensating for the
orientation of a vessel relative to the direction of movement. The
seismic cables are coupled to rigid beams the orientation of which
are changed using wires when the vessel for example has to be
directed against the wind or current. It does, however, not provide
any possibility for compensating for deviations in the vessels
position from the predetermined path.
It is an object of the present invention to provide a simple system
using existing navigation systems for compensating for drift and
movements relative to a predetermined course, without making large
demands to the manoeuvring systems of the vessel and to the systems
treating the seismic data. It is also an object of the present
invention to provide a system using simple and commercially
available equipment position on the vessel.
It is a further object of the invention to provide a system and a
method making it possible to maintain the seismic cables in a
linear movement even if the towing vessel must perform manoeuvres
departing from the predetermined direction within certain
limits.
These objects are obtained using a system and a method as disclosed
in the accompanying independent claims.
The invention will be described in detail below with reference to
the accompanying drawings, illustrating the invention by way of
examples.
FIG. 1 illustrates movements of a seismic survey vessel according
to the known art.
FIG. 2 illustrates the movements of a seismic survey vessel
according to the invention.
FIG. 1 illustrates how a seismic survey vessel 1 according to the
known art may move under influence of different wind and current
conditions. The vessel 1 is coupled to a seismic array 2, 3, 4, 8
comprising towing cables 2, 8 and deflectors 3 stretching a tow
sideways relative the towing direction, and seismic cables 4
comprising sensors, such as hydrophones. In addition the tow
comprises seismic sources 5, which in this case are coupled
directly to the vessel 1.
In FIG. 1 the vessel is supposed to follow a predetermined course
6, but because of wind and current conditions the real movement 7
will deviate from this. The control system compensating for the
deviations will usually result in an oscillating movement around
the predetermined course, which is illustrated in the vessels
positions A, B, and C in the drawing. The seismic array, being
coupled to the vessel with towing cables, lead-ins or similar 2, 8
having a fixed length, will follow the movements of the vessel.
In FIG. 2 the vessel is provided with devices changing the length
of the towing cables as a response to changes in the position of
the vessel relative to the predetermined course 6. These devices
may be any kind of available equipment, e.g. winches, and will not
be described in any detail here. One example of a possible winch is
the Scancontrol 2000 system from Scandinavian Control Systems AS,
Norway, being mainly used in fishing trawlers. In some cases
already existing winches for taking in or out the towing cables,
lead-ins or similar may be used, when they are suitable for
connecting to a control system.
The vessel is in a usual manner equipped with navigation systems,
e.g. the GPS (Global Positioning System), which may provide a
measure of the deviations from the predetermined course, and which
may be used to compensate for the variations in the movements of
the vessel. As it is significantly easier and faster to pull in or
let out the towing wire the position of the array may be adjusted
faster than the vessels position. Thus the towed seismic array,
represented by the deflectors 3 in positions B and C, may keep a
more stable course than the towing vessel itself.
In position B in FIG. 2 the vessel 1 is at the left of the
predetermined course. To compensate for this the towing cable 2 is
shortened, thus moving the deflector closer to the vessel. In a
similar manner the right towing cable is let out, so that the right
deflector increases its distance to the vessel 1.
In position C in FIG. 2 the vessel is to the right of the
predetermined path, and the towing cables are compensated by making
the right cable shorter and the left cable longer.
For exact control over the seismic arrays position the vessel may
be provided with devices for measuring the positions of one or more
of the array parts, e.g. the deflectors, relative to the vessel.
Then the measured deviation in the vessels position may be
compensated directly by letting out or pulling in a sufficient
amount of cable until the deflector has the correct position, both
globally and relative to the vessel.
Alternatively the cable length being let out or pulled in is
adjusted as a function of the angle .theta. between a line from the
vessel to the deflector and the predetermined direction of movement
and the deviation from the vessels predetermined position.
In case of deviations being perpendicular to the predetermined
course the cable length may be adjusted with ##EQU1##
.DELTA.k being the change in the cable length and a.sub.195 being
the measured deviation perpendicular to the direction of
movement.
If the position of the vessel deviates in the direction of
movement, e.g. due to changes in the velocity, this may be
compensated in the same way according to ##EQU2##
a.sub..vertline. being the deviation parallel to the predetermined
course.
Preferably this compensation is done simultaneously with all the
cables, lead-ins etc being coupled to the system. This may of
course also be done in combination with compensation for the
transversal deviation a.perp..
In practice these simple models must be adjusted relative to the
towing resistance of the towed seismic array.
As is evident from FIG. 2 the compensation for the transversal
deviation will lead to a relative displacement of the deflectors in
the direction of movement, thus changing the relative position of
the seismic cables. If this makes a problem in the data acquisition
the change may be compensated for in different ways, e.g. by
adjusting the lengths of the seismic cables or by using active
deflectors increasing their lift so that the variation in the used
cable length is less while the deflector lift capability is used to
keep a more even position relative to the towing vessel in the
direction of movement. One example showing such an active deflector
is for example disclosed in international patent application No.
PCT/NO97/00302.
For increased precision the heave, roll and pitch movements of the
vessel may be measured and compensated for according to the
invention.
In the preferred embodiment of the invention the system comprises a
deflector on each side of the vessel, in which the adjustment at
least comprises the towing cables, but may also include the rest of
the wires, cables, or lead-ins 8 in the shown embodiment and
stretching to each of the seismic cables 4. This will also provide
a possibility for adjusting the seismic cables position in the
direction of movement for compensating for the displacement which
otherwise will follow from the change in the length of the towing
cable 2 to the deflector 3.
The drawings shows a towed array where the seismic cables 4 are
connected to each other and to the deflectors with cables 9 thus
providing the required distance between the cables. It is of course
possible to split the tow into a number of parts, each being
coupled to one or more deflectors, and where the position of each
of these parts may be adjusted to keep the towed seismic array in a
predetermined course.
Even if the drawings show a symmetrical tow it is of course
possible to the invention with asymmetrical arrays, for example in
cases where a number of vessels together creates a towed seismic
array. One solution when the whole array is towed on one side of
the vessel may also be contemplated, e.g. in surveys close to the
shore, where the vessel must be kept at a certain distance from
land.
Even if the invention is primarily described as a means for
compensating for drift relative to the predetermined course it is
clear that it also may be used for keeping the the seismic array
along a predetermined path while the vessel for example manoeuvres
to avoid obstructions. For example it may be used in rivers where
the sailable path is not necessarily straight.
* * * * *